. . . "rginine, lysine, phenylalanine, and tryptophan, and X3 is an amino acid selected from the group consisting of serine, threonine, alanine, tyrosine, leucine, histidine, asparagine, and glutamine, and X4 is an amino acid selected from the group consisting of cysteine, homo-cysteine, penicillamine, histidine, tyrosine, asparagine, glutamine, and methionine, and X5 is an amino acid selected from the g" . . .